Pluto's 3:2 Resonance with Neptune

Pluto is locked into a 3:2 resonance with Neptune. For every 3 orbits of the Sun completed by Neptune, Pluto completes 2 orbits. At any given moment, however, the ratio is not exactly 3:2. Sometimes Pluto's period is slightly faster than its average value. Sometimes it is slower.

When Pluto's period is slightly faster than average, the points where its orbit intersects the orbit of Neptune advance with each orbit. But when this intersection gets too close to Neptune, Pluto is accelerated by Neptune's gravity. This causes Pluto to rise into a higher orbit with a longer period. Now orbiting the Sun with a period slightly slower than its average value, the points where Pluto's orbit intersects Neptune's orbit retreat with each orbit. Eventually, it approaches Neptune from the other direction, allowing Neptune's gravity to pull Pluto into a lower orbit with a shorter period. This repeats indefinately, ensuring that Pluto and Neptune never get too close to each other.

The simulation PlutoResonance.gsim shows the orbit of Pluto in a rotating frame whose period matches the period of Neptune. This causes Neptune to appear stationary, exposing this 3:2 resonance. The orbits of Jupiter (purple), Saturn (yellow), and Uranus (green) are also visible.

The starting conditions for this simulation were obtained by JPL's Horizons Ephemeris Computation Service.